1. Field of the Invention
The present invention relates generally to rate responsive cardiac pacemakers, and more particularly to an implantable rate-responsive pacemaker wherein two or more sensors indicative of physiological demand are utilized in a fashion designed to realize the advantages of both sensors in a circuit producing a rate command signal which is used to operate the pacemaker at an optimum pacing rate which will closely match physiological need of the patient.
A pacemaker is an implantable medical device which delivers electrical stimulation pulses to a patient's heart in order to keep the heart beating at a desired rate. Early pacemakers provided stimulation pulses at a fixed rate or frequency, such as 70 pulses per minute (ppm), thereby maintaining the heart beat at that fixed rate. Subsequently, pacemakers were designed to not only stimulate the heart, but also to monitor the heart. If a natural heart beat was detected within a prescribed time period (usually referred to as the "escape interval"), no stimulation pulse was delivered, thereby allowing the heart to beat on its own without consuming the limited power of the pacemaker or interfering with the normal operation of the heart. Such pacemakers are referred to as "demand pacemakers" because stimulation pulses are provided only as demanded by the heart.
Early demand pacemakers had a fixed base rate associated with them. In later versions, the base rate was programmably selectable, and thereafter became commonly known as the "programmed rate." If the heart was able to beat on its own at a rate exceeding the base (or programmed) rate, then no stimulation pulses were provided. However, if the heart was not able to beat on its own at a rate exceeding the base rate, then stimulation pulses were provided to ensure that the heart would always beat at least at the base (or programmed) rate. Such operation was achieved by simply monitoring the heart for a natural beat during the escape interval. If natural activity was sensed, the timer which defined the escape interval was reset. If no natural activity was sensed, a stimulation pulse was provided as soon as the escape interval had timed out. Changing the base (or programmed) rate was accomplished by simply changing the duration of the escape interval.
In recent years, rate-responsive pacemakers have been developed which automatically change the rate at which the pacemaker provides stimulation pulses as a function of a sensed physiological parameter. The physiological parameter provides some indication of whether the heart should beat faster or slower, depending upon the physiological needs of the pacemaker user. Thus, for example, if a patient is at rest, there is generally no need for a faster-than-normal heart rate, so the rate-responsive pacemaker maintains the "base rate" at a normal value, such as 60 pulses per minute (ppm).
However, if the patient is exercising, or otherwise physiologically active, there is a need for the heart to beat much faster, such as, for example, 100 beats per minute. For some patients, the heart is not able to beat faster on its own, so the pacemaker must assist. In order to do this effectively, the physiological need for the heart to beat faster must first be sensed, and the "base rate" of the rate-responsive pacer must be adjusted accordingly. Hence, rate-responsive pacemakers are known in the art which increase and decrease the "base rate" as a function of sensed physiological need.
Numerous types of sensors are taught in the art for use with a rate-responsive pacer. In each, an increase or decrease in the parameter being monitored signals a need to increase or decrease the rate at which pacing pulses are provided. Note, as used herein, the term "pacing rate" refers to the rate at which the pacer provides stimulation pulses, or in the case of demand pacers, the rate at which the pacer would provide stimulation pulses in the absence of naturally occurring heart beats.
One common type of sensor is an activity sensor which senses the physical activity level of the patient. See, for example, U.S. Pat. No. 4,140,132, to Dahl, and U.S. Pat. No. 4,485,813, to Anderson et al. In accordance with the teachings of Dahl or Anderson et al., a piezoelectric crystal is used as an activity sensor. Such a crystal generates an electrical signal when subjected to physical movement and stress according to well known principles. The electrical signal generated by the crystal may be processed and used to vary the pacing rate.
Other types of sensors used in prior art rate-responsive pacers include sensors that sense respiration rate, respiratory minute volume, blood oxygen level, blood and/or body temperature, blood pressure, the length of the Q-T interval, the length of the P-R interval, etc. All of the sensors which may be used in rate-responsive pacers have particular advantages and disadvantages.
The next generation of rate-responsive pacemakers will use two or more sensors simultaneously to control the pacing rate. It will be appreciated by those skilled in the art that the combination of signals from two or more sensors to be used to control pacing rate is a difficult and complex task.
The goal of a system using two or more sensors should be to utilize the best properties of each of the sensors, while eliminating or minimizing their drawbacks. For example, an activity sensor will react very quickly to the onset of exercise, closely mimicking the response of the sinus node in a healthy heart. However, an activity sensor does not measure any true physiological variable of the body, and as such may be a poor predictor of work level and of the optimum heart rate.
Alternately, a sensor measuring respiratory minute volume or venous blood temperature will provide a very good correlation to the level of exercise at higher levels of exercise. However, the sensor response of a respiratory minute volume sensor or venous blood temperature sensor is much slower than the response of the SA node, typically of the order of sixty to ninety seconds. Thus, it may be seen that all single sensor systems will have both significant advantages and disadvantages.
Theoretically, a combination of an activity sensor and a respiratory minute volume sensor or venous blood temperature sensor could be used to control pacing rate in a manner which is more physiologic than either of the sensors separately. The combination technique may, however, prove quite complex in its implementation. For example, a summation and averaging of the two signals would not be optimum for the following reasons. At the onset of exercise, the activity sensor would deliver a signal, while the other sensor would not yet have reacted. Thus, the onset of heart activity would be slower than in the case of using activity alone.
During prolonged exercise, the good sensor response of the respiratory minute volume or blood temperature sensor would be averaged with the poorer response of the activity sensor. In this case, the result would not be as accurate as using the respiratory minute volume or blood temperature sensor alone. During prolonged exercise at a low level, the activity sensor may be as good as the respiratory minute volume or blood temperature sensor because the latter two are inaccurate at low levels of exercise. In the case of a false positive indication of activity of the activity sensor (caused, for example, by riding in a car on a bumpy road), the poor response of the activity sensor would be averaged with the good response of the respiratory minute volume or blood temperature sensor. Thus, the result again would not be as accurate as using the respiratory minute volume or blood temperature sensor alone.
Another possible technique which may be used to combine the inputs from two sensors would be to take the highest value of the two sensors. This would in at least some cases yield a better result than the averaging technique discussed above. This combination would not, however, eliminate the erroneous increase in pacing rate resulting from external vibration picked up by the activity sensor.
Thus, while this technique could give a better response to exercise in some situations, it would not eliminate problems occurring due to erroneous responses of the sensors. In addition to the problem of external vibration mentioned above, if the other sensor used was a blood temperature sensor, the shortcomings of this sensor would be propagated. For example, heavy clothing or external temperature change would result in erroneous changes to the pacing rate. In short, it will be perceived by those skilled in the art that it is difficult and complex to utilize inputs from more than one sensor in an intelligent fashion which will enhance the advantages of each sensor without proliferating the drawbacks of the sensors.
It is accordingly the objective of the present invention that it provide a system which will utilize inputs from two or more sensors to provide a sensor-indicated rate signal, which will control the pacing rate of the pacemaker. It is the primary objective of the system of the present invention to utilize the best properties of each of the sensors, while minimizing or eliminating their drawbacks. The control strategy must be of a complexity sufficient to provide as an output a highly flexible sensor-indicated rate signal which will accurately follow a control strategy paralleling the physiological response of a healthy heart.
It is a further objective of the present invention that its implementation be relatively simple and easy to accomplish in a pacemaker, which is necessarily limited in size since it is an implanted device. The system of the present invention should be useable with at least two sensors, but should also be capable of working with more than two sensors. The system should also be economic of power, not requiring more power to operate than do other rate response systems. Finally, it is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.